October 25, 2005


History of Earthquakes in the United States

By Dr. Frank J. Collazo


Introduction: The attachment describes the scope of damage from the 1989 and 1906 earthquakes of San Francisco.  Furthermore, an explanation of the type of earthquake, intensity, magnitude, Ritcher Scale Metrics and Modified Mercalli Scale, and causes of the earthquakes are provided.


Earthquake Overview:  Shaking of the Earth’s surface caused by rapid movement of the Earth’s rocky outer layer.  Earthquakes occur when energy stored within the Earth, usually in the form of strain in rocks, suddenly releases.  This energy is transmitted to the surface of the Earth by earthquake waves.  The study of earthquakes and the waves they create is called seismology (from the Greek seismos, “to shake”).  Scientists who study earthquakes are called seismologists.  Earthquake NBC News Archives/ABCNews VideoSource explains the destruction an earthquake causes depending on its magnitude and duration, or the amount of shaking that occurs.  A structure’s design and the materials used in its construction also affect the amount of damage the structure incurs.


Earthquakes vary from small, imperceptible shaking to large shocks felt over thousands of kilometers.  Earthquakes can deform the ground, make buildings and other structures collapse, and create tsunamis (large sea waves).  Lives may be lost in the resulting destruction.  Archive articles include Ø1938: Seismology Ø1939: Seismology Ø1940: Seismology Ø1941: Seismology Ø1942.  Earthquakes, or seismic tremors, occur at a rate of several hundred per day around the world.  A worldwide network of seismographs (machines that record movements of the Earth) detects about 1 million small earthquakes per year. 


Very large earthquakes, such as the 1964 Alaskan earthquake, which caused millions of dollars in damage, occur worldwide once every few years.  Moderate earthquakes, such as the 1989 tremor in Loma Prieta, California, and the 1995 tremor in Kobe, Japan, occur about 20 times a year.  Moderate earthquakes also cause millions of dollars in damage and can harm many people.  In the last 500 years, several million people have been killed by earthquakes around the world, including over 240,000 in the 1976 T’ang-Shan, China, earthquake.  Worldwide, earthquakes have also caused severe property and structural damage.  Adequate precautions, such as education, emergency planning, and constructing stronger, more flexible, safely designed structures, can limit the loss of life and decrease the damage caused by earthquakes.


Focus and Epicenter Earthquake Expand:  The point within the Earth along the rupturing geological fault where an earthquake originates is called the focus, or hypocenter.  The point on the Earth’s surface directly above the focus is called the epicenter.  Earthquake waves begin to radiate out from the focus and subsequently form along the fault rupture.  If the focus is near the surface—between 0 and 70 km (0 and 40 mi) deep—shallow-focus earthquakes are produced.  If it is intermediate or deep below the crust—between 70 and 700 km (40 and 400 mi) deep—a deep-focus earthquake will be produced.  Shallow-focus earthquakes tend to be larger, and therefore more damaging, earthquakes.  This is because they are closer to the surface where the rocks are stronger and build up more strain.


Locating Epicenters:  Seismologists can locate the epicenter of an earthquake by triangulation, a method that involves taking seismographic measurements from at least three separate seismic stations.  Seismologists measure the time it takes seismic waves to reach the recording stations, as well as the magnitude of the waves, and triangulate the measurements to calculate the location of the epicenter.


Tectonic Earthquakes:  Tectonic earthquakes are caused by the sudden release of energy stored within the rocks along a fault.  The released energy is produced by the strain on the rocks due to movement within the Earth, called tectonic deformation.  The effect is like the sudden breaking and snapping back of a stretched elastic band.


Modified Mercalli Scale:  Scale for measuring the intensity of earthquakes, adapted from the original Mercalli scale.  The Mercalli scale was devised in 1902 by Italian seismologist Giuseppe Mercalli.  American seismologists Harry O. Wood and Frank Neumann created the Modified Mercalli scale in 1931 to measure the intensity of earthquakes that occur in California.  The Modified Mercalli scale, or a scale similar to it, is now used worldwide.  The scale has 12 levels of intensity.  Each level is defined by a group of observable earthquake effects, such as shaking of the ground and damage to structures such as buildings, roads, and bridges.  The levels are designated by the Roman numerals I to XII. 


Levels I through VI are used to describe what people see and feel during a small to moderate earthquake.  Levels VII through XII are used to describe damage to structures during a moderate to catastrophic earthquake.  On average, about one earthquake of level X to XII occurs worldwide every year; 10 to 20 earthquakes of level VII through IX occur each year; and over 500 earthquakes of level I to VI occur every year.  Each year over 100,000 earthquakes occur that are not noticed by the human population and therefore are not rated on the Modified Mercalli scale.


Earthquake intensity is a measure of the effects of an earthquake in a particular place. Scientists who study earthquakes, known as seismologists, do not need special equipment or instruments to use an intensity scale.  Seismologists can use recorded observations and intensity scales to compare the sizes of earthquakes that have occurred throughout history.  However, they cannot use intensity scales to measure earthquakes that occur on the ocean floor because there are no people on the ocean floor to observe the effects of earthquakes there.


Seismologists use other scales to classify the magnitude of earthquakes.  Magnitude is a measure of the strength of an earthquake, or the amount of strain that rocks in Earth’s crust release when an earthquake occurs.  The Richter scale and the Moment Magnitude scale are used to measure the magnitude of earthquakes.  Seismologists use sensitive instruments called seismographs to measure earthquake magnitudes and patterns and to locate the source of an earthquake.  Seismographs also help to locate an earthquake’s epicenter, the point on the surface of the earth directly above the earthquake’s focus, the point within the earth where an earthquake originates.  Seismologists even put seismographs in places where there are no people to observe the effects of earthquakes.


The surface effects of an earthquake, such as shaking of the ground, lessen with distance from the epicenter of the earthquake.  Hence, an earthquake’s Modified Mercalli rating depends on where the earthquake is measured.  Close to the epicenter of a moderate earthquake, ground motion is felt by nearly everyone. Landslides occur on steep slopes, ground cracks are widespread, and some buildings and other structures suffer significant damage.  Farther from the epicenter, fewer people feel the earthquake.  Level I on the Modified Mercalli scale is defined as "not felt except by a very few under favorable conditions.”


Seismologists use level I to describe very small earthquakes.  At the other end of the scale, level XII describes very large, catastrophic earthquakes that cause “total destruction.”  Earthquakes of intensities II and III are roughly equivalent to earthquakes of magnitude 3 to 4 on the Richter scale.  Intensity levels XI and XII on the Modified Mercalli scale are similar to magnitudes 8 to 9 on the Richter scale.


The main flaw of the Modified Mercalli scale is its subjectivity, or its reliance on the opinions of human observers.  No instruments are used to measure ground motion. Instead, seismologists who use the Modified Mercalli scale gather information after an earthquake by means of letter questionnaires sent to earthquake victims or reports from the local population.  After assigning Mercalli numbers to points in the affected areas, a seismologist draws contours, called isoseismal lines on a map to separate places of equal intensity.  The pattern of isoseismal lines indicates the regions of greatest shaking and provides evidence that can be used to locate the epicenter.


If geological conditions such as soil composition and rock structure are similar near the epicenter, the isoseismal lines create a uniform, circular pattern.  However, isoseismal lines are usually irregular in shape because the geology and soil conditions of the affected areas vary.


Richter Scale:  Method of ranking the strength or size of an earthquake.  The Richter scale, also known as the local magnitude scale, was devised in 1935 by the American seismologist Charles F. Richter to rank earthquakes occurring in California.  Richter and his associates later modified it to apply to earthquakes anywhere in the world.


The Richter scale ranks earthquakes based on how much the ground shakes 100 km (60 miles) from the earthquake’s epicenter, the site on the earth’s surface directly above the earthquake’s origin.  An instrument called a seismograph measures the amount of ground movement.  Seismographs can detect movements as small as about 0.00001 mm (about 0.000004 inches) to movements as large as about 1 m (about 40 inches).  In order to deal with numbers in such a broad range, the Richter scale is a logarithmic scale—each increase of 1 on the Richter scale represents a tenfold increase in movement. Thus, an earthquake registering 7 on the scale is 10 times as strong as an earthquake registering 6, and the earth moves 10 times as far.


Earthquakes of magnitude 5 are considered moderate, while quakes of magnitude 6 are considered large, quakes of magnitude 7 are considered major, and quakes of magnitude 8 or larger are considered great.  For example, the Los Angeles earthquake of 1994 was a magnitude 6.7 earthquake, and the San Francisco earthquake of 1906 was a magnitude 7.9 earthquake. Although there is no upper limit to the Richter scale, earthquakes of magnitude 8 or greater are rare.  Worldwide, they occur only about once a year.


Scientists believe that the crust cannot store enough energy to release a magnitude 10 earthquake.  There is also no lower limit on the Richter scale.  An earthquake one-tenth the size of a magnitude 1 earthquake would be a magnitude 0 earthquake, and an earthquake one tenth that size would be a magnitude -1 earthquake.  Earthquakes with negative Richter scale magnitudes occur every day, but are so small that they are difficult to detect.


The amount of energy released by an earthquake is related to how much the earth moves. The energy released by an earthquake increases 32 fold for each increase of 1 on the Richter scale.  Thus, an earthquake registering 7 on the Richter scale releases 32 times as much energy as an earthquake registering 6, even while the earth moves only 10 times as far.  The amount of energy released by a magnitude 4.3 earthquake is equivalent to the energy released by the atomic bomb that destroyed Hiroshima, Japan, which is equivalent to about 20 kilotons of TNT.  The largest earthquakes recorded to date measured about 9.5 and released as much energy as 66,000,000 Hiroshima-sized atomic bombs.  It is estimated that a magnitude 12 earthquake would release enough energy to split the earth in half.


The Richter scale is only one of several scales used to measure earthquakes.  Currently, the scale most commonly used by seismologists to rank the effects of earthquakes is the Modified Mercalli Intensity Scale, or MM scale.  The MM scale measures the effects of an earthquake at different sites and thus the same earthquake has different MM scale values at different sites.  The MM scale is marked from I (for barely detectable) to XII (for almost complete destruction).


Seismographs Deployment: There are more than 1,000 seismograph stations in the world.  One way that seismologists measure the size of an earthquake is by measuring the earthquake’s seismic magnitude, or the amplitude of ground shaking that occurs. Seismologists compare the measurements taken at various stations to identify the earthquake’s epicenter and determine the magnitude of the earthquake.  This information is important in order to determine whether the earthquake occurred on land or in the ocean.  It also helps people prepare for resulting damage or hazards such as tsunamis. When readings from a number of observatories around the world are available, the integrated system allows for rapid location of the epicenter.  At least three stations are required in order to triangulate, or calculate, the epicenter. 

Seismologists find the epicenter by comparing the arrival times of seismic waves at the stations, thus determining the distance the waves have traveled.  Seismologists then apply travel-time charts to determine the epicenter.  With the present number of worldwide seismographic stations, many now providing digital signals by satellite, distant earthquakes can be located within about 10 km (6 mi) of the epicenter and about 10 to 20 km (6 to 12 mi) in focal depth.  Special regional networks of seismographs can locate the local epicenters within a few kilometers.


Causes:  April 1906 California earthquake.  He proposed the elastic rebound theory to explain the generation of certain earthquakes that scientists now know occur in tectonic areas, usually near plate boundaries.  This theory states that during an earthquake, the rocks under strain suddenly break, creating a fracture along a fault.  When a fault slips, movement in the crustal rock causes vibrations.  The slip changes the local strain out into the surrounding rock.  The change in strain leads to aftershocks (smaller earthquakes that occur after the initial earthquake), which are produced by further slips of the main fault or adjacent faults in the strained region.  The slip begins at the focus and travels along the plane of the fault, radiating waves out along the rupture surface. 


On each side of the fault, the rock shifts in opposite directions.  The fault rupture travels in irregular steps along the fault; these sudden stops and starts of the moving rupture give rise to the vibrations that propagate as seismic waves.  After the earthquake, strain begins to build again until it is greater than the forces holding the rocks together, then the fault snaps again and causes another earthquake.


Magnitude of the Earthquake: The 1906 San Francisco earthquake and the fires it caused claimed more than 3000 lives and 28,000 buildings.  Estimated at 7.9 on the Richter scale, the earthquake still ranks as one of the largest in world history.  Following the quake, the city’s residents worked together to quickly rebuild San Francisco.


1906 Earthquake Shock: This stunning account was one of the first to tell the rest of the nation about San Francisco's devastating earthquake.  The quake and the resulting tidal wave and fire destroyed nearly all of San Francisco's city center.  With the entire north section of the city in ruins and with the flames leaping from building to building in all directions, San Francisco seems doomed.  Unless the wind shifts to the west and blows the flames towards the bay, nothing can prevent the destruction of the city.  The Fire Department, working frantically without water, is dynamiting building after building in the path of the flames, but the wind is carrying a roaring river of fire across each gap and it appears impossible to check the conflagration.


One by one the finest structures in the business section are being reduced to wreckage. Every building surrounding the Palace Hotel is in flames.  Fire is eating its way into the 16-story building of San Francisco Call, a morning paper, and the rear section of the 11-story Monadnock Building has collapsed, spreading the fire in all directions.  The Postal Telegraph Co. is preparing to vacate its building, and this will shut off all telegraphic communication with the outside world.


The death list is added to every moment.  Aside from those that lost their lives nearly 1500 are injured, it is estimated.  It is utterly impossible to care for the wounded as they should be, and many are lying in the streets breathing their last, with the people in their madness unable to get them to places of safety.  Men, women and children with broken limbs can be seen vainly trying to reach medical aid.  Physicians from Oakland, Berkeley, Alameda and San Rafael have arrived on the scene and are doing good work in caring for the injured.


With no water to fight the flames and the town being gradually consumed and the moaning and cries of the injured, the city has been thrown into a panic.  The awful scenes of dead bodies lying around on the streets have caused widespread horror.  The Waterworks is destroyed.


Earthquake Shock:  San Francisco, April 18, 10:15 am:  There has just been another shock, which intensified the panic.  People have started to rush into the streets, but the shock was of short duration and alarm subsided.   The gas works, south of Market Street, has blown up and an immense fire rages in that vicinity.  The fire in the vicinity of the Palace and Grand hotels is rapidly approaching these buildings and from present indications they will fall prey to the flames within half an hour.


Tsunami:  Tsunami is a Japanese word, meaning “harbor wave” and used as the scientific term for seismic sea wave generated by an undersea earthquake or possibly an undersea landslide or volcanic eruption.  When the ocean floor is tilted or offset during an earthquake, a set of waves is created similar to the concentric waves generated by an object dropped into the water.  Most tsunamis originate along the Ring of Fire, a zone of volcanoes and seismic activity, 32,500 km (24,000 miles) long that encircles the Pacific Ocean.  Since 1819, about 40 tsunamis have struck the Hawaiian Islands.


A tsunami can have wavelengths, or widths, of 100 to 200 km (60 to 120 miles), and may travel hundreds of kilometers across the deep ocean, reaching speeds of about 725 to 800 km/h (about 450 to 500 mph).  Upon entering shallow coastal waters, the wave, which may have been only about half a meter (a foot or two) high out at sea, suddenly grows rapidly.  When the wave reaches the shore, it may be 15 m (50 feet) high or more. Tsunamis have tremendous energy because of the great volume of water affected.  They are capable of obliterating coastal settlements.


Tsunamis should not be confused with storm surges, which are domes of water that rise underneath hurricanes or cyclones and cause extensive coastal flooding when the storms reach land.  Storm surges are particularly devastating if they occur at high tide.  A cyclone and accompanying storm surge killed an estimated 500,000 people in Bangladesh in 1970.


1906 Earthquake of San Francisco: Earthquake Hazards in the Western United States. During the first week of July the U.S. Geological Survey (USGS) released a report with revised estimates of the probabilities of major earthquakes in California.  According to the report, different segments of the San Andreas Fault, the most studied in California, have quake probabilities during the next 30 years that range from almost none to nearly 100 percent.  The most scrutinized segments of the fault are those lying close to major population centers—Los Angeles, whose last major quake, of magnitude 8, struck in 1857, and San Francisco, whose 7.5 or greater quake happened in 1906.  The USGS experts concluded that a major earthquake of magnitude 7.5 or more has a 60 percent chance of occurring in the Los Angeles area within the next 30 years, 10 percent higher than the forecast of a study released in 1980.  The USGS group also stated that for Southern California as a whole, the chance of a magnitude 7.5 quake within 30 years now stands at a whopping 70 percent.


In Northern California, the scientists singled out the East Bay communities of San Francisco, such as Hayward, Fremont, Oakland, and Berkeley, which straddle the Hayward fault.  This fault stands a 20 percent chance of a major shock within the next 30 years; if the probabilities for the Hayward fault are combined with the probabilities for the nearby segment of the San Andreas fault, the San Francisco Bay area stands a 50 percent chance of a major shock.  Such a quake could kill 1,500 to 4,500 people and injure more than 45,000, according to a recent California study. 


The USGS scientists made their earthquake predictions on the basis of calculations involving the average number of years between quakes along specified lengths of the fault.  The same week they released their findings, scientists from the Lamont-Doherty Geological Observatory in Palisades, N.Y., and the California Institute of Technology published a report that raised questions about this methodology and suggested that earthquake probabilities for California are more uncertain than had been previously thought.  The Lamont-Doherty and Caltech scientists had searched for evidence of earthquake damage in the annual growth rings of trees near the San Andreas Fault at Wrightwood, California. 


In pines, firs, and cedars, the scientists identified rings that indicated damage from very large quakes.  They then worked out the approximate years these rings grew.  They found that in addition to the great 1857 quake, major earthquakes had struck the Los Angeles area around 1812 and 1480.  Since the interval between the 1812 and 1857 tremors, it was only about 45 years and between the 1480 and 1812 quakes was about 330 years.   They concluded that earthquakes in Southern California strike at highly irregular intervals.  In view of these findings, the researchers cautioned that relying on average recurrence times of quakes is an insufficient prediction method, and that accurate prediction of large quakes on the San Andreas requires a more fundamental knowledge of the dynamics of the fault.

1989 Loma Prieta Earthquake: Eighteen kilometers beneath the San Andreas Fault, two enormous plates of earth's crust had been locked in a planetary pushing match since the great San Francisco earthquake of 1906.  These players were tiring, reaching the breaking point.  The two sides of the San Andreas shot past each other simultaneously and the west side of the fault rose, lifting the mountains themselves.


The ripping was unstoppable.  For about eight seconds earth's crust unzipped at more than two kilometers a second, 20 kilometers to the north and south.  The bucking Santa Cruz Mountains flicked several houses off their foundation, cracking them like an eggshell.  The faulting released a frenzy of seismic waves.  They set seismometer needles scribbling around the world and carried a lethal message to Californians.


Waves rolling to the south bludgeoned the city of Santa Cruz, only 16 kilometers from the epicenter.  They took out its commercial heart and snuffed four lives.  The waves smashed into Watsonville, damaging or destroying most homes and turning Main Street into a ghost town.  They mutilated Hollister and churned the rich sediments of the Salinas Valley.


Waves rolling north roiled the ground beneath picturesque Los Gatos, shattering Victorian houses and half the business district.  They shook San Jose, but most buildings held.  The waves swept up the peninsula, rattling securely planted cities such as Palo Alto and Menlo Park.  At Stanford University they found old, brittle structures and twisted and cracked them.  Ahead lay Candlestick Park, packed with 62,000 fans and ripe for disaster. The waves shook the people of the community and other bewildered spectators.  But Candlestick sits on bedrock, and it defeated the waves.


Now the waves were weakening.  With little effect they jiggled southern San Francisco and towns across the bay.  A tiring vanguard of waves reached San Francisco's old Market Street area and Marina district and Oakland's busy waterfront.  These areas sit on man-made fill.  Here the waves found soil in tune with their own vibrations and strummed it like a guitar string.  More waves arrived and pumped in more energy.  The earth grew alive and danced.  The vibrations flowed upward into buildings and highway structures.  Picking up the rhythm, soil and structures swayed to the strengthening beat like partners in a dance.


Marina buildings buckled; many fell.  Column joints supporting Oakland's Interstate 880 failed, and 44 slabs of concrete deck, each weighing 600 tons, collapsed on cars below. The waves pushed the Oakland end of the Bay Bridge 18 centimeters to the east, and a 15-meter section crashed onto the level beneath.  Within 15 seconds the vibrations faded. But 63 persons lay dead or dying.  Some 3,800 others suffered injuries requiring medical attention.  The waves damaged more than 24,000 houses and apartment buildings as well as nearly 4,000 businesses.  At least a thousand structures faced demolition.


Measured in adjusted dollars, property damage approached that of the dreadful temblor of 1906, which unleashed 60 times as much energy.  The Loma Prieta damage exceeded that inflicted by Hurricane Hugo during the hours it lashed the Southeast.  Still, California had been lucky.  A few more seconds of shaking could have severed a crucial joint of San Francisco's battered Embarcadero Freeway, bringing it crashing down like I-880, and thousands more homes would have been damaged or destroyed.  If bolts had not failed on the Bay Bridge, swaying trusses could have pulled down more of the vital span.


With the many wounds, moreover, came a new sense of confidence among Californians, a belief that they are doing many things right about quakes.  A few of the pluses: The relatively low level of damage.  “Keep in mind that the vast majority of bay area buildings suffered no damage,” emphasized John Osteraas of Failure Analysis Associates, Inc., a Menlo Park engineering firm, the value of preparedness.


Within hours of the earthquake, shelters opened from the Marina district to Hollister. Though staffed partly by legions of spontaneous volunteers, these nerve centers had been carefully planned.  Throughout the year the Red Cross, the state Office of Emergency Services, and other agencies conduct rehearsals that bore fruit in the October 17 response.


The reliability of quake forecasts grew.  A 1988 assessment of earthquake probabilities along the San Andreas Fault, published by the U. S. Geological Survey, had assigned the southern Santa Cruz segment the highest likelihood of slipping for northern California.

“Loma Prieta strengthens our confidence that our simple models are accurate enough to be useful,” said the Survey's Allan Lindh, a leading force in earthquake prediction.


The human response, like an opened spigot, the quake released an untapped flood of caring and kindness.  Volunteers materialized as if from the shaking earth, directing traffic on darkened streets, comforting the terrified with a word and a hug, extricating the injured and the dead.  One saw it everywhere, from the war zone of the wealthy Marina district to the shambles of blue-collar Watsonville.  “Nearly 400 of us are helping here,” said Lynne Newhouse, a Red Cross volunteer at the Marina Middle School.  The concrete structure swarmed with homeless and helpers.


Lynne lives in Pacific Heights, on the precipitous hills above the Marina.  Surely those precariously perched houses had slid in the shaking.  But no; the good rock of the hills had resisted the waves, and she had escaped with only a brief loss of electric power.

In the school gymnasium cots covered a wooden floor marked out with two basketball courts.  Homeless victims sat on their cots or in chairs ringing the courts.


The Marina was past the water crews restoring the area's 66 broken mains and past streams of evacuees towing wheeled suitcases full of clothes.  Some houses were intact but had partially sunk into the soil.  Beside them was a small volcano-shaped pile of sand, a sign that the shaking had liquefied the soil allowing the building to settle.  The Marina’s first-floor garage level had cracked, and it leaned gently against a similar row house.  The garage level, several of its spindly wooden supports had shattered.  Row houses were sagging above buckled garage levels ... buildings settling amid tiny sand volcanoes.  The pattern of destruction repeated itself over and over in the Marina district.


Engineering Failures:  “Each of these problems,” said Tom Hanks of the U. S. Geological Survey in Menlo Park, “arose from the fact that man made the geology as well as the structures—and did both poorly.  The same applies to Oakland.  “The hardest hit area of the Marina district didn't exist until California put on the great Panama-Pacific International Exposition in 1915.  The Panama Canal opened the year before, and San Francisco wanted to advertise itself as a Pacific port—to show it had rebuilt after the 1906 earthquake and fire.


“Engineers used 1906 rubble to fill in the shallow water of the Marina district for exposition buildings.  They pumped in mud and sand and didn't properly compact it. Seismically that's the least stable soil you can think of.”  When seismic waves reached the Marina fill, the shaking stirred the soil and groundwater into a thick soup that behaved like quicksand—a phenomenon known as liquefaction.  Buildings gently settled.

Structures too were built to fail.


“Throughout the city,” said Bernard Ross of Failure Analysis, “houses and apartment buildings are built with garages taking up the first level.  This means many open walls, often with slender supporting columns for the levels above.  We call this a soft story.  The highest toll occurred among corner structures, which had few adjacent buildings to lean against.”


Santa Cruz: The venerable seaside city had been cruelly mauled.  In the heart of town, piles of rubble and teetering walls wrote an epitaph for the Pacific Garden Mall.  Until the quake this lovingly restored area had pulsed as the heart of city commerce and pride. Like the Marina, the mall sat on unstable ground, in this case river sediment.  Here in Santa Cruz the theme of destruction took new turns.  Enormous slides, triggered by the quake, closed the road to general traffic.  Road crews would toil a month to clear 435,000 tons of rock, soil, and trees and open the vital link between Santa Cruz and San Jose.


“Many of the damaged buildings stood against each other and were of different heights,” said Helmut Krawinkler of Stanford University.  “This gave them different periods of vibration and caused them to butt against one another.  Some pounded each other to ruins.  Sometimes the shorter building, with the quicker movement, dislodged the taller building's sidewall, and it fell on the short structure.  Masonry buildings that were not reinforced are death traps and should be reinforced whenever possible.”


Insurance: Insurance ... good rock ... bolting down the house: These are buzzwords in post-quake California.  Once notoriously uninterested in earthquake insurance, Californians have changed their ways.  “Policy purchases have skyrocketed in the past 15 years,” said Risa Palm of the University of Colorado in Boulder.  In a study of four California counties completed a year ago, she found that about 30 percent of homeowners were insured.  Since 1985, state law has required insurance companies to offer quake coverage with homeowner policies—by certified mail to assure receipt.  Dr. Palm credits this with part of the increased sales.


Who buys quake insurance?  Not necessarily those who live near known faults or on unsafe soils.  “The buyers,” said Dr. Palm, “are those with the greatest earthquake awareness—people who perceive the risk.”  Californians in increasing numbers are investing in another form of house insurance: simple structural reinforcement.  “A homeowner can bolt the home to its foundations and strengthen the crawl space with plywood for about $500” said Peter Yanev of EQE, Inc., the nation's largest earthquake engineering firm.  “A contractor will do it for $2,500.”


Like insurance and reinforcement, the value of living on bedrock came home on October 17.  “The safest place you can be,” remarked Robert Brown of the USGS, “is on level ground that is bedrock.  Unfortunately, California doesn't have enough of either.”  Can Californians learn what lies beneath their dwellings?  “The word has been out for a long time,” said the Survey’s Roger Borcherdt.


Seismic Intensity Maps: In 1975 Kenneth Lajoie and James Gibbs published a seismic-intensity map for the bay area, based partly on Dr. Borcherdt's analysis of soil reactions to seismic waves from underground nuclear tests in Nevada.  Their map has had a lot of impact.  Local jurisdictions have incorporated versions into land-use plans.  Los Angeles and other areas have followed suit.  But development pressures continue.  Dr. Borcherdt worries about high rises invading Redwood Shores, a community built on a compacted fill south of San Francisco. “The fill has a natural seismic period of about one second—the same as a ten-story building.  These could shake violently in a large quake.”  The soil lessons of Loma Prieta echo the quake experiences of Armenia in 1988, Mexico City in 1985.


Armenia and Mexico Earthquakes: More than 25,000 died in Armenia; unofficial estimates double that figure.  Mexico City's temblor claimed 10,000 victims.  In both locales the devastated buildings sat atop deep lake and river’s sediment that amplified the waves as much as ten times.


Watsonville: Low on a plain in the Pajaro Valley lay Watsonville, a frozen-vegetable processing center.  Here damage rivaled the Marina's, but the misery was incalculable. Many of Watsonville's distressed were poor: Hispanic migrant workers and laborers in the food plants.  I stopped at a shelter.  “A lot of our people live in ramshackle houses, garages, and shoddy low-income housing,” said Carlotta Woolcock, a manager of a farm workers association.  “They suffered worst.”


Ms. Woolcock presided calmly over a scene reminiscent of D-day.  A succession of trucks unloaded food, clothing, and bedding donated from around the state and nation. human chains passed the goods to impromptu depots.  Lines of victims picked up the goods.


Across the road, National Guard helicopters ferried in more supplies.  Behind the shelter, a tent village held families whose houses were no longer habitable or who avoided a roof overhead for fear of aftershocks.  Some had acquired this fear four years earlier in Mexico City, where a powerful aftershock demoralized stricken inhabitants.


Main Street, Watsonville's commercial core, was eerily deserted. Stores showed cracked facades, fallen gables, smashed showcase windows.  At the north end St. Patrick's Church stood wounded, hemmed with fallen brick.  “That's bad for this community, so strongly Catholic,” said Yolanda Ortega, whose bungalow had buckled but stood.

Oakland: The gray hulk of collapsed I-880 slumped in its rubble, a somber headstone for a part of civilization that failed.  That concrete pancake had trapped 58 vehicles, some flattened to less than a foot high.  “The excavators dug down to a car roof, and firemen peeled it back.  The victims were extricated and their personal possessions.  Some bodies were crushed and dismembered.  Built to the earthquake-engineering criteria of the 1950s, later strengthened in part but never completely, I-880 today stirs a tempest of fault finding and priority-sorting.  There is little dispute, however, about the treachery of the soil it stood on.


The intensity map prepared by Borcherdt, Lajoie, and Gibbs identifies the ground as fill over bay mud and predicts “violent” shaking.  A field test conducted by seismologists at Columbia University's Lamont-Doherty Geological Observatory showed that the freeway structure vibrated at the same frequency as the underlying mud, which amplified the wave motion as much as eight times.


What kind of quake would deliver such a strong punch 95 kilometers from its source?  By most geologic measures Loma Prieta had been a strange beast.  The following is a summary of its peculiarities:


Depth of Rupture. The hypocenter, or point of first slip, was 18 kilometers down, instead of the usual 10 or 12 on the San Andreas.  “The mountain may sit atop a downward bulge of the crust,” said the Survey's Robert Burford.


Lack of Surface Rupture: Quakes of this size usually gash the surface in their path. Loma Prieta scarified the mountains with cracks, but no surface cleft marked the rupture, although it extended underground for 40 kilometers.  “Perhaps the surface was inelastic, like sand, and did not respond to the motion beneath,” theorized Will Prescott of the USGS.


Quick Release of Energy: The rupturing lasted only about eight seconds, brief for so large a temblor.  “This is because it spread bilaterally, from the center outward to both sides,” said Bruce Bolt of Berkeley.  The 1906 quake, which began near the Golden Gate Bridge, ripped the peninsula for a full minute.


Analysis: Unusual thrust.  Along the San Andreas, geologists expect the two sides of the fault to slip past each other horizontally, the west side moving to the northwest.  This movement totaled about two meters.  But the west side also moved vertically, riding up on the east side between one and two meters.  “It caused the Santa Cruz Mountains to grow at about ten times the rate we normally associate with mountain building,” said Dr. Prescott.


Lack of surface rupture disturbs geologists.  “In the past we identified quake activity by surface faulting,” said Kenneth Lajoie.  “If a large quake can leave no geologic record, we could be living on top of hazards we don't know of.”


Like the tail of a comet, thousands of after-shocks followed Loma Prieta.  The most severe—a magnitude 5.2—terrified residents only 37 minutes after the main shock.  As this article went to press, 24 had registered magnitude 4 or stronger.  (On February 28 an unrelated quake measuring 5.5 rattled southern California.)


Aftershocks and foreshocks intrigue scientists.  So do micro-quakes, the faint seismic chatter that emanates from many faults.  “These pulses illuminate the tectonics below,” said William Ellsworth of the USGS—”if you're there to see them.”  To “see” them, seismologists set out arrays of seismo-meters.  Some 700 eavesdrop on California's web of faults, checked by the Survey, University of California, Caltech, and others.


The seismic signals may be telling geologists exactly where major quakes will occur. Hear David Oppenheimer of the Survey: “Micro-quakes occur along a fault as a result of stress buildup.  Over time we see that their activity leaves one area quiet.  Foreshocks behave the same way—their hypocenters appear anywhere but in that quiet area.  That quiet area is where the sides are locked; that is where the earthquake will occur.”


Forecast: He sees this pattern emerging on the Rodgers Creek Fault, a little-known but potentially dangerous extension of the Hayward Fault north of Oakland.  The likely magnitude is 7.  When?  “The time aspect is the wild card,” said Mr. Oppenheimer. “We're working now on a carbon-14 date for the last quake on Rodgers Creek.  That could give a rough idea.”


The timing of quakes is indeed the big unknown.  Nevertheless, scientists have calculated earthquake probabilities for the various segments of the San Andreas.  Each fault segment has its own quake chronology.  This includes the size of the last quake and when it occurred—information often revealed by fault-line excavations pioneered by Kerry Sieh of the California Institute of Technology (CALTECH).  These geologic histories, melded with instrument data, enabled scientists to calculate the probabilities.  The time brackets for these forecasts are wide.  For the section ruptured by the Loma Prieta earthquake, the probability was 30 percent in the next 30 years.


Imprecise, certainly, but even such gross forecasts can be helpful to long-term planners such as earthquake engineers.  Are precise predictions on the way?  Fifteen years ago many earth scientists thought so.  But the quarry has proved elusive.  A major obstacle is to discover where a quake might occur, so instruments can be set out to record the quake's precursors.


The discovery of a dependable quake fell to the Survey's Allan Lindh and William Bakun, and Thomas McEvilly of the University of California at Berkeley( UCB).  Its lair is Parkfield, a ranching community on the San Andreas midway between San Francisco and Los Angeles.  Every 22 years or so the people of Parkfield are hammered by a quake of magnitude 6.  And another is due any moment.  With Thomas Burdette of the Survey, I took a look at the world's most densely instrumented quake trap.  “Whatever has been reported as a possible precursor to a quake, we have an instrument set to record it here,” he said.  We passed by seismometers, accelerometers, and creepmeters; strainmeters, tiltmeters, and magnetometers; leveling lines and geochemical sensors.


Carl Hill:  On the right the San Andreas carved a narrow trough; I leaped across it, from the Pacific plate to the North American.  At the top the laser can train on other hills bounding the 600-square-kilometer experiment.  Three times a week Duane Hamann, the teacher at Parkfield's one-room school, ascends to measure surface movements less than half the thickness of a dime.  We drove past Middle Mountain, epicentral area of the Parkfield quakes.  The earth beneath us trembled—the quake!  But it was only the shaker truck, a UCB’s machine that pounds the earth so scientists can look for changes in wave travel time—possible precursor of a quake.


A University of Alaska experiment monitors the level of “white noise,” earth's normal background electromagnetic activity.  Japanese and Soviet scientists have reported strong fluctuations preceding quakes.  Such changes could be related to reports of unusual animal behavior—a phenomenon that has often been reported but not scientifically verified.  Did Loma Prieta give precursors?  There are several candidates, with varying credentials.


Eleven days before the quake, a radio receiver in the Santa Cruz Mountains detected the onset of low-frequency signals about 30 times stronger than normal.  Three hours before the quake the signals shot off the instrument's scale.  “They certainly seem associated with the quake,” said investigator Antony Fraser-Smith of Stanford.  “The cause could relate to currents generated as a result of stress in the rock.  The USGS has persuaded me to set up a similar test in Parkfield.”


Interest also focuses on two magnitude 5 quakes that occurred near the northern end of the Loma Prieta rupture in August 1989 and June 1988.  Both episodes triggered advisories warning of increased hazard of earthquakes.  The understanding of those foreshocks could lead the way to more accurate predictions.


Enthusiasm is growing for a type of earthquake warning espoused by Thomas Heaton of the U. S. Geological Survey in Pasadena.  It is based on the fact that radio waves travel much faster than earthquake waves.  In this real-time process a major earthquake activates a seismometer.  Instantly a warning signal flashes by satellite to distant receivers, giving notice tens of seconds before the destructive waves arrive.  Time for students to duck under desks, computers to be turned off, nuclear power plants to respond.


Today prediction plays little role in plans for coping with quakes.  Instead, emphasis is on preparedness.  Observers generally agree that emergency-service organizations responded well to Loma Prieta.  Unquestionably, preparedness training in California has accelerated in recent years.  Each April the state observes the anniversary of the 1906 disaster with Earthquake Preparedness Month.  Schools are required by law to conduct rehearsals twice a year.  Utilities and other businesses have intensified emergency responses and the bracing of buildings.  Many show increasing concern for the earthquake safety of employees' families.


Will the area rebound?  Surely so, if it sustains the spirit of the merchants and townsfolk of Santa Cruz.  With their mall destroyed, storeowners erected tents behind shattered buildings and stocked them for Christmas shoppers.  Slow recovery is seen for hard-hit areas such as Watsonville, where the prolonged need for emergency services strains facilities already stressed to their limits.  Has the bay area seen the Big One?  Geologists are virtually unanimous that it has not.  Observed William Ellsworth: “Loma Prieta was merely a shot across our bow.”


Historical Earthquakes:  Widely differing magnitudes have been computed for some of these earthquakes; the values differ according to the methods and data used.  For example, some sources list the magnitude of the February 7, 1812 New Madrid quake as high as 8.8. Similar variations exist for most events on this list, although generally not so large as for the example given.


Ten Largest Earthquakes in the World by Magnitude Since 1900


           Location                                            Date                     Magnitude

Chile May



Prince William Sound, Alaska



Andreanof Islands, Aleutian Islands



Kamchatka Peninsula



Off the Coast of Ecuador       



Rat Islands, Aleutian Islands  



India-China Border



Kamchatka Peninsula



Banda Sea, Indonesia



Kuril Islands




The Largest Earthquakes on Record in the Contiguous United States:


         Location                             Date                      Time             Magnitude

New Madrid, MO


9:45 am


Old Fort Tejon,


4:24 pm


Owens Valley, CA


     10:30 am


Imperial Valley, CA


7:20 am


New Madrid, MO


8:15 am


San Francisco, CA


       1:12 pm


Pleasant Valley,  NV


6:52 am


New Madrid,  MO


3:00 pm


Landers, California


     11:57 am


Kern, California


     11:52 am


West of Lompoc, CA


       1:50 pm


Dixie Valley, NV


     11:07 am


Hebgen Lake, MO


6:37 am


Borah Peak, Idaho


2:06 pm




Largest Recorded U.S. Earthquakes

Location                                    Date                     Time


Prince Wm Sound, AK


3:36 am


Andreanof Islands, AK


2:22 pm


Rat Islands, Alaska


5:01 am


East of Shumagin


8:18 pm


  Islands, Alaska




Lituya Bay, AK


6:15 am


Yakutat Bay, AK


5:04 pm


Near Cape Yakataga


       12:22 am


Andreanof Islands, AK


       10:47 pm


New Madrid, MO


         9:45 am


Fort Tejon, CA


         4:24 pm


Ka'u District, Hawaii


         2:25 am


Kodiak Island, AK


        12:25 pm


Gulf of Alaska


          7:23 pm


Owens Valley, CA


        10:30 am


Imperial Valley, CA


          7:20 am